1. Field of the Invention
The invention relates to a process for preparing hydrogenalkoxysilanes of formula I EQU H.sub.n Si[O(CH.sub.2).sub.m CH.sub.3 ].sub.4-n (I),
where n is an integer from 1 to 3 and m can assume the value 0, 1 or 2, by direct reaction of metallic silicon with alcohols in the presence of suitable, copper-containing catalysts.
2. Discussion of the Background
Hydrogenalkoxysilanes are an important group of inorganic silicon compounds. Via the Si--H functional groups, hydrosilylation processes make available organosilanes which can usually only be obtained by esterification of chlorosilanes. Furthermore, monosilanes can be obtained in high purity, for example for semiconductor applications, by base-catalyzed dismutation of hydrogenalkoxysilanes.
It is known that compounds of formula I can be prepared by esterification of the corresponding hydrogenchlorosilanes or, more elegantly, not employing chlorosilanes, by direct reaction of metallic, pulverized silicon with the corresponding alcohols at temperatures above 100.degree. C. using copper-containing catalysts (e.g. U.S. Pat. No. 2,473,260, U.S. Pat. No. 3,641,077, DE 258 961). The industrially and economically most interesting processes use inert heat-transfer oils as reaction media. Here, the copper-containing silicon contact composition is suspended in an inert, liquid medium and is converted into the desired hydrogenalkoxysilane at temperatures above 100.degree. C. by passing a liquid or gaseous alcohol into the reaction medium. The maximum temperature at which this process can be used is determined by the thermal stability of the heat-transfer oil used.
The main byproducts formed in the known direct synthesis are tetraalkoxysilanes and siloxanes. Tetraalkoxysilanes are formed mainly by direct reaction of alcohol with silicon or by reaction of hydrogenalkoxysilanes with alcohol with liberation of hydrogen. Siloxanes are formed mainly by hydrolysis of the alkoxysilanes. The water required for this can originate in dissolved form from the alcohol used or is formed thermally or catalytically by elimination of water from ethanol with formation of ethylene.
In the past, various reaction liquids have been used for the reaction discussed here. On studying the literature, it is conspicuous that the selectivity of the reaction and the utilizability of the silicon used (Si conversion) are dependent on the quality of the inert reaction medium employed. Thus, for example, when silicone oil is used as the reaction medium, a selectivity of 77% is obtained in the preparation of triethoxysilane and only 50% for trimethoxysilane with low total utilization of the Si used (U.S. Pat. No. 3,641,077). This process has been improved by use of alkylbenzenes (EP 0 280 517, U.S. Pat. No. 4,931,578 giving 88.2% selectivity and 81% Si conversion in the synthesis of trimethoxysilane, with the disadvantage that methyl chloride has to be used to activate the contact composition). An oil comprising aromatic hydrocarbons having from 2 to 4 rings and 1&lt;n&lt;4 alkyl groups having less than 4 carbon atoms of formulae C.sub.10 H.sub.8-n R.sub.n, C.sub.12 H.sub.10-n R.sub.n [A petroleum cracking product which contains sulphur, which acts as a catalyst poison,] gives 94% selectivity with the disadvantage that hydrogen fluoride has to be used to activate the silicon (DE 2 247 872).
Furthermore, use has been made in the past of the following inert reaction media:
diphenyl oxide (JP 1002693),
isoparaffin mixtures (JP 57108095),
dodecylbenzene (JP 57108094, JP 3027493, JP 3156793),
ditoluenes (JP 54163529),
partially hydrogenated terphenyls (Therminol 59, U.S. Pat. No. 5,084,590, BP 0 462 359).
Although the compounds mentioned are readily available industrially (heat-transfer oils), they do not allow industrially satisfactory values for selectivity and silicon conversion to be obtained. This is the main reason why the process for the direct reaction of metallic silicon with ethanol or methanol has not yet been used on a large industrial scale, although there is an urgent need for this, since the .alternative process (esterification of chlorosilanes) is technically complicated. It is furthermore known that the use of certain heat-transfer fluids leads to foaming problems in carrying out the reaction (Comparative Example 3 in DE 2 247 872), which could have catastrophic consequences on a large industrial scale.
A need has therefore continued to exist for reaction media which result in improved conversion of silicon to product while reducing foaming of the reaction mixture.